Spineless Sticklebacks

Seafaring stickleback fish sport a pair of prominent spines sticking out from their pectoral fins. Their freshwater cousins look far less threatening, outfitted with much smaller spines. Now researchers have found that a simple change of gene activity could make all the difference--a rare demonstration of how a small genetic change can make a relatively rapid impact on an organism.

Sticklebacks started out as saltwater fish. As the glaciers of the last Ice Age receded 10,000 years ago, some of these fish swam inland, and many became trapped in lakes left behind by the glacial melt. Over time, the lake fish came to look quite different from their marine forefathers. These populations are now natural laboratories for evolutionary studies.

One prominent difference is the pelvic spines. Marine fish have prominent spines, presumably to deter predators, but the freshwater fish have only small ones, if any at all. That's probably because their new environment was short of the minerals needed to make the spines, which made spine-building consume too much of the fishes' energy to be worthwhile. By breeding saltwater and freshwater fish, David Kingsley, a vertebrate geneticist at Stanford University, and his colleagues have tracked down the genetic basis for the loss of the spines, showing that the spines didn't simply disappear for lack of raw materials. The breeding studies first helped his team home in on parts of the genome responsible for spine formation. They then began looking at genes in the region that seemed particularly influential and focused on a gene called Pitx1.

By using genetic markers to follow Pitx1 expression during development, they found that it was active in the incipient pelvic girdle of marine sticklebacks but not in the freshwater fish. Elsewhere in the bodies of the marine fish, Pitx1 was just as active as in freshwater fish, suggesting that a change in the gene's regulation--and not in the gene itself--caused the lake sticklebacks to lose their spines, the team reports in the 15 April issue of Nature. As further support they noted that mice with no Pitx1 activity have smaller than normal hind limbs and are asymmetrical, just like the freshwater stickleback.

The study is important, because it shows that “relatively simple genetic changes can have profound effects,” says R. Craig Albertson, an evolutionary biologist at the Forsyth Institute in Boston, Massachusetts.